Switch the is_integral_or_enum trait machinery to use an explicit

Switch the is_integral_or_enum trait machinery to use an explicit
template argument and an *implicit* conversion from '0' to a null
pointer. For some bizarre reason, GCC 4.3.2 thinks that the cast to
'(T*)' is invalid inside of an enumerator's value... which it isn't but
whatever. ;] This pattern is used elsewhere in the type_traits header
and so hopefully will survive the wrath of the build bots.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152220 91177308-0d34-0410-b5e6-96231b3b80d8